1. Field of the Invention
This invention relates to a process for the production of an ion exchange membrane and more particularly, it is connected with a process for producing a heterogeneous ion exchange membrane having a more improved ion transport number in a high ion concentation as well as a more improved specific resistance in an aqueous polyvalent ionic salt solution then the heterogeneous ion exchange membrane of the prior art.
2. Description of the Prior Art
An ion exchange membrane comprising an ion exchange resin attached to a support of synthetic resin screen has been put to practical use as a semi-homogeneous membrane, but there are various limitations in the practical use thereof. That is to say, this semi-homogeneous ion exchange membrane shows a remarkably different degree in swelling when it is dipped of aqueous solutions and shrinkage when it is natrually dried in the air, whereby the membrane becomes deformed and cracked and cannot be put to practical use. Therefore, the ion exchange membrane is stored in a wet state, which necessity is a bar to practical use thereof. In addition, since such an ion exchange membrane generally lacks flexibility, there are some difficulties when using this ion exchange membrane in an electrodialysis apparatus.
For the purpose of solving these problems, it has hitherto been proposed to prepare a heterogeneous ion exchange membrane by mixing a finely powdered ion exchange material with a polyolefin as a matrix, molding the mixture and subjecting it to a post-treatment using hot water. For example, there are a method comprising kneading an ion exchange resin with polypropylene, shaping the mixture in the form of a film and treating with an acid and alkali (U.S. Pat. No. 3,627,703) and a method comprising treating a formed film with hot water (U.S. Pat. No. 3,876,565).
However, the heterogeneous ion exchange membranes prepared by these methods can be put to practical use with some advantages in the desalting of an aqueous solution having a relatively low ion concentration, but are not very satisfactory in the desalting of an aqueous solution having a relatively high concentration. That is, in an aqueous solution having a high ion concentration, the heterogeneous ion exchange membrane prepared by the above described method shows an adequately low specific resistance, but has at the same time a disadvantage that the ion transport number is considerably lowered. In the heterogeneous ion exchange membranes, microcracks are formed due to swelling of the ion exchange membrane during the post-treatment with hot water, which is a control factor of the ion exchange membrane. When using an ordinary crystalline thermoplastic resin as a matrix, stress concentration points tend to occur in the inner part against the swelling pressure because of little molecular chains becoming entangled with each other and large microcracks are thus formed. It is assumed that the water contained therein with a high ion concentration lowers the ion transport number.
In a process for the production of a heterogeneous ion exchange membrane having a high ion transport number in a high ion concentration, therefore, it is essential to decrease the diameters of microcracks. To this end, it is considered to decrease the diameters of microcracks and to raise the ion transport number by increasing the degree of cross linking of the ion exchange resin and decreasing the degree of swelling of the ion exchange resin. However, this method has a disadvantage that the specific resistance is increased although the ion transport number is increased and the durability, e.g. chemical stability, is improved.
The inventors have hitherto made various efforts to overcome these disadvantages of the prior art and consequently, have invented a process for the production of a heterogeneous ion exchange membrane which ion transport number is not lowered in a high ion concentration, which process comprises kneading a finely powdered ion exchange material with a bridge-making and film-making polymer, heating and pressing the resultant mixture to form a film, simultaneously or thereafter bridging the above described polymer and then subjecting the formed film to a treatment with hot water (Japanese patent application (OPI) No. 88678/1978).